At seabed resource development sites, underwater actuators for performing various work under seawater are used. For example, Patent Literature 1 discloses an underwater separator. In Patent Literature 1, in order to collect an underwater measurement device moored to a weight after underwater observation has been done, the underwater separator causes the underwater measurement device to release a rope connected to the weight, thereby separating the underwater measurement device from the weight. The underwater separator is configured to drive a motor to move a hook-fixing pin to a releasing position. As a result, a hook that is holding the rope is released from a fixed state, and the hook rotates downward about a support pin due to its own weight.